1. Field of the Invention
This invention relates to methods of and apparatus for component separation of composite digital video signals.
2. Description of the Prior Art
A composite video (television) signal comprises luminance and chrominance components. Two color difference signals that make up the chrominance signal (U/V signals in the case of PAL and I/Q signals in the case of NTSC) are modulated in quadrature on a color subcarrier which has a frequency (e.g. 4.43 MHz for PAL and 3.58 MHz for NTSC) towards the upper end of the frequency band or spectrum (e.g. 0 to 5.5 MHz) occupied by the luminance component. The modulated sub-carrier is then added to the luminance component to produce the composite signal. Since the chrominance component occupies an upper portion of the band occupied by the luminance component, crosstalk of luminance into chrominance (known as "cross-color") and of chrominance into luminance (known as "cross-luminance") can occur when the composite signal is separated into its luminance and chrominance components.
Traditionally, a composite signal is separated into its luminance and chrominance components by simple filtering techniques. Thus, a simple notch filter, which passes the whole of the spectrum of the composite signal except for a "notch" region centered on the color subcarrier frequency, because most chrominance energy is concentrated around the color subcarrier frequency, is used to separate the luminance component. However, the notch filter will pass the sidebands of the chrominance signal and thereby allow some of the chrominance to pass through to appear as crosstalk (cross-luminance) on the luminance, the extent of the crosstalk depending on the color content of the picture. This form of crosstalk, arising due to imperfect separation of chrominance from luminance, can manifest itself on a displayed picture in the form of a dot effect at edges of brightly colored objects, this cross effect being known as "subcarrier crawl". A simple bandpass filter, which is centered on the color subcarrier frequency and has a passband of a width similar to that of the band occupied by the chrominance component in the composite signal, is used to separate the chrominance component from the composite signal. Since this filter will pass any luminance energy which is of a high enough frequency to appear within its pass band, it will allow some of the luminance to appear as crosstalk (cross-color) on the chrominance, the extent of the cross-color depending upon the luminance content of the picture. By way of example, a striped pattern on, for instance, an article of clothing can, if more than a certain distance from a video camera, have a spatial frequency which will give rise to a high frequency luminance component which appears as cross-color in the separated chrominance signal and can manifest itself on a displayed picture in the form of a cross effect characterized by a colored rainbow effect superimposed on the distant striped pattern.
The relatively crude separation technique employing simple filters, as described above, is not good enough for use in high quality component video systems, as used for example in broadcasting.
A better degree of luminance/color separation than that obtainable by using simple filtering can be achieved by using comb filtering, which takes advantage of the frequency interleaving that takes place between the luminance and chrominance components of the composite signal. Such frequency interleaving is possible because the spectral characteristics of both the luminance and chrominance energy are determined by the horizontal and vertical scanning of the video signal. Thus, as is well known in the art, the luminance energy within the composite signal is not continuous across the spectrum. Instead, it is concentrated within the spectrum (e.g. 0 to 5.5 MHz) at positions at the line frequency and harmonics thereof, each such concentrated spectral position having sidebands on opposite sides thereof, the spacing between each such pair of sidebands being equal to the field frequency. Little energy is present in the gaps or slots between the line harmonics. The relationship between the line frequency and the color subcarrier frequency (in the case of both PAL and NTSC) is, as is well known in the art, chosen so that the chrominance information (which is distributed in a similar manner to the luminance information) is concentrated at positions between the harmonics of the line frequency (where the luminance energy is concentrated) across the region of the spectrum (the upper end) occupied by the chrominance component. In simpler terms, the chrominance information is frequency interleaved with the luminance information by putting the chrominance information into the gaps or slots between the luminance information towards the upper end of the spectrum of the composite signal.
A comb filter for separating the chrominance component from the luminance component has plural adjacent passbands, the peaks of which are positioned appropriately (in the frequency domain) within the above-mentioned gaps (and the nulls between the peaks of which are positioned to coincide with the line frequency harmonics at which the luminance energy is concentrated) whereby the filter "combs out" the chrominance energy from the gaps between the luminance energy. Conversely, to separate luminance from chrominance, the nulls are positioned appropriately within the gaps (and the peaks coincide with the line frequency harmonics) whereby the filter "combs out" the luminance energy from between the chrominance energy.
In the case of digital composite signals, where digital comb filters are employed, use may be made of vertical (line delay) comb filters, vertical/temporal (field delay) comb filters or temporal (frame delay) filters. Vertical comb filters act within a field of the signal between vertically adjacent samples in different lines of the field. They effect a combing action as just described above. They provide a degree of luminance/chrominance separation which, in general, is much better than that provided by simple filtering. A vertical/temporal (field delay) filter provides a degree of separation which, in general, is even better than that provided by a vertical filter, in that it acts between samples in different fields and performs a finer combing action between interleaving field sidebands that are disposed around the concentrated regions of luminance and chrominance energy that appear in the spectrum. Vertical/temporal filters act between respective consecutive fields, whereby they have a characteristic in the frequency domain which is similar to that of vertical filters except that the peak locations are determined by the field frequency rather than by the line frequency. A temporal (frame delay) filter, which acts in a purely temporal sense, between samples in different frames, is (in principle) even better.
Comb filters in general provide a considerably better degree of component separation (and thus a lesser degree of intercomponent crosstalk) than the simple form of filtering described above, which ignores the frequency interleaving of the component separation and treats the spectrum as if it were continuous. Comb filters are thus better suited to obtaining a high quality interface between composite and component video systems. Nonetheless, crosstalk can occur when comb filters are employed. In this regard, the amount of energy in the gaps or slots in the spectrum between the luminance information are picture-dependent. Thus, if there is a lot of luminance energy in the picture, the slots will tend to fill with luminance energy, to a degree dependent on the picture content, whereby luminance energy will crosstalk into the chrominance regions to provide cross-color. Also, if there is a lot of chrominance energy in the picture, the slots will tend to fill with chrominance energy, to a degree dependent on the picture content, whereby chrominance energy will crosstalk into the luminance regions to provide cross-luminance. The inventors have ascertained that such crosstalk can arise due to high vertical energy in the picture, for example due to a vertical transition (i.e. a change of luminance and/or (in particular) chrominance in the picture in the vertical direction within a field, which change is not necessarily associated with movement) and/or due to high vertical/temporal energy in the picture (e.g. due to a picture change between fields caused by movement).
Since a temporal (frame delay) comb filter acts purely temporally, while it provides good results for a static picture it is highly susceptible to crosstalk in the case of picture movement. The inventors have ascertained that, in general, vertical/temporal (field delay) comb filtering provides a better degree of separation than either vertical (line delay) comb filtering or simple filtering, not only in the case of a static picture but also, in general, in the case of a picture having a moderate amount of movement and a moderate amount of vertical energy. However, in the event of more severe picture movement, vertical/temporal picture energy due to such movement can cross fairly readily into the passbands of a vertical/temporal comb filter, whereby crosstalk (in particular cross-color) can arise in these circumstances when component separation is being obtained by way of a vertical/temporal comb filter.
The inventors have also ascertained that, in practice, a vertical (line delay) comb filter can break down in the case of high vertical energy (e.g. due to vertical chrominance transitions), in that high vertical picture energy can cross fairly readily into its passbands. However, it is not in general so susceptible to movement as a vertical/temporal (field delay) comb filter. (The direction of movement is relevant: it is not susceptible at all to movement along the temporal axis, the degree of susceptibility thus increasing as the direction of movement goes away from that axis.) Further, a vertical/temporal (field delay) comb filter is not so susceptible as a vertical (line delay) comb filter to high vertical energy (e.g. due to vertical transitions) whereby, for example, in the case of vertical/temporal filtering, failure at vertical chrominance transitions is much less evident than in the case of vertical filtering.
UK Patent No. 2 163 023 (Sony Corporation), which is hereby incorporated herein by reference, discloses apparatus for component separation of a composite video signal, the apparatus comprising means for measuring crosstalk between luminance and chrominance components of the video signal, a vertical comb filter for separating the chrominance component from the composite signal, a vertical/temporal comb filter for separating the chrominance component from the composite signal, and means for switching between the vertical filtering and vertical/temporal filtering in response to the crosstalk measured by the crosstalk measuring means so as to reduce cross-effects resulting from the crosstalk.
The switching preferably was smoothed in order that it would occur no more than once every few samples, rather than (as is possible) on a sample by sample basis, with the intention of avoiding unacceptable switching artifacts (i.e. subjectively unacceptable visible phenomena) in the displayed picture. Subject to this limitation, switching between different modes of filtering was effected frequently with the intention of providing the best possible picture. In practice, it has been found that frequent switching, in spite of the smoothing, does give rise to unacceptable switching artifacts. Further, another difficulty arose. This was that certain spatial frequencies gave rise to an uncertainty in the algorithm controlling the switching (in accordance with measured crosstalk) whereby an ambiguity as to which mode of filtering to adopt arose. This gave rise to repetitive switching (as often as the smoothing action would permit) at certain picture boundaries, producing unacceptable picture artifacts.